Knee arthritis and trauma in various forms can cause loss of joint cartilage, including for example, osteoarthritis, excessive wear or sudden trauma, rheumatoid arthritis, or infectious arthritis. When joint cartilage is worn away, the bone beneath the cartilage is left exposed, and bone-on-bone contact can be very painful and damaging. Other types of problems can occur when the bone itself becomes diseased. One conventional solution for these types of joint problems takes the form of total knee replacements. In a total knee replacement (TKR), the proximal end of the tibia is replaced with a tibial component, the distal end of the femoral bone is replaced with a femoral component, and the patella is replaced with a patellar component. Such procedures often require sacrifice of the anterior and posterior cruciate ligaments.
However, many patients who develop knee arthritis experience issues isolated to the medial (inner) compartment and the patellofemoral (knee cap) part of the joint, while the lateral (outer) compartment of the joint remains healthy. The conventional treatment for such patients is either the combination of a unicompartmental knee in conjunction with a patellofemoral implant or the use of a total knee implant, which requires removal of the healthy lateral condyle. However, one recent solution is a hybrid femoral component that preserves the healthy lateral condyle as well as the anterior and posterior cruciate ligaments, and only replaces the medial compartment and patellofemoral joint. (Such a hybrid femoral component may be used in conjunction with a unicompartmental tibial tray, which only requires resurfacing of part of the tibia as well). A hybrid femoral component requires a smaller incision and preserves ligaments that can help the knee retain its natural kinematics. It can be implanted using a procedure called a bicompartmental knee replacement.
A bicompartmental knee replacement is a procedure that replaces only the medial (inner) parts of the femoral and tibial components. It does not resurface or resect the lateral parts of the knee (including the distal femoral articular cartilage), and as such, can allow the anterior and posterior cruciate ligaments to be retained. Bicompartmental knee replacements have a number of advantages over total knee replacements. Because the outer lateral portion of the joint is not resurfaced, the incision made may be smaller, resulting in less pain, quicker recovery time, and less blood loss. Also, because certain ligaments do not need to be sacrificed, a greater stability of the knee can be maintained.
The femoral component used in such a replacement is often called a monolithic implant. It has an anterior portion and a medial condyle portion, without a lateral condyle portion (again, because as much of the lateral bone as possible is retained). As with most typical femoral implants, the component may be made of titanium, stainless steel, cobalt-chrome, zirconium, oxinium, any combination thereof, or any other appropriate material that has sufficient strength and biocompatibility for use in knee replacement surgery.
While performing bicompartmental knee replacement with a monolithic implant, it is necessary to locate the implant on the bone properly, in order, among other things, to achieve proper articulation in both the medial and lateral compartments of the knee between femur and tibia, as well as proper articulation between the patella and the femur or femoral component interface. For example, the surgeon wants to retain as much healthy bone as possible while removing the diseased bone, but also needs to consider the depth of the medial condyle portion of the implant in order to ensure that there is a smooth transition from the implant to the bone and to maintain proper performance of the reconstructed knee in flexion and extension.
With conventional patellofemoral replacements, one popular current method for preparing the bone to receive an implant is to use an osteotome in conjunction with a trochlea trial to mark the boundary of the transition between the implant and the bone. However, there is no known solution or method for marking the boundary for bicompartmental knee replacement. Accordingly, such surgeries are conventionally performed using traditional total knee replacement instrumentation, without any additional components that help identify certain reference points. For example, recessing the implant to the cartilage on the lateral side is important, and without specific instrumentation or techniques for this type of procedure, the surgeon is left to estimate the cuts that are needed.